Why do greasy meals upset my stomach?

Via the Q&A section of our site we got the following question: "Why do greasy meals upset my stomach?"

It's a good question, and surprisingly in doing a cursory google search, it's pretty interesting what kind of disinformation is out there. It was nearly impossible to find any kind of scientific, straight forward explanation of why greasy foods give people gastrointestinal (GI) issues. Before we begin, it may be helpful to review some basics about how the gut works - it's always good to get a refresher on some of the vocabulary we will use.

There are two things I want to clarify about the question. First, for the purposes of our conversation I'm going to equate "greasy" foods with fatty and often fried foods. Next, "upset stomach" is a little vague, and defining it brings to mind two distinct processes: heartburn and diarrhea. Let's start with heartburn.

Heartburn (aka reflux or GERD) is one of the most commonly experienced GI complaints. Part of the process of a healthy stomach breaking down your food is allowing some of its acidic gastric juice to "reflux" into your esophagus. Actually being able to feel that sensation leads to heartburn in 20% of adults. While certain foods can certainly precipitate symptoms, what causes heartburn is complex, and will be the subject of a future post.

Fried foods (along with spicy foods and alcohol to name a few others) have long been known to precipitate of heartburn. Why? Fried, fatty foods are more difficult for your stomach to break down. The longer these foods have to hang around the stomach, and presumably the more acid it will take to break them down, the higher likelihood of that acid refluxing into your esophagus, giving you symptoms. Fatty foods are also thought to loosen the ring of muscle (aka sphincter) that prevents stomach acid from reaching your esophagus in the first place, making matters worse.

Everything we've talked about so far relates to your "upper" GI tract. Now that it's time to delve into why fatty foods cause diarrhea, we are going to be discussing the "lower" GI tract - namely, the colon. As discussed in our article on GI basics, your colon is where your food will remain for the longest amount of time, and is responsible for allowing water in and out of its tract to form your stool. To that end (pun intended), there are a few immediate reasons that come to mind as to why greasy foods tend to give people diarrhea. The first is straight forward: greasy, fried foods and the salt load they inevitably contain are going to attract a lot of water inside the tube of your colon. If you imagine your colon as a column that lets water freely travel in and out of it, more salt content drawn into the tube means that ultimately it only has one place to go: out the other end. That may speed up the time that food is spent in your colon, thus increasing the water content of your stool.

It's pretty taxing for your gut to digest fatty foods. Your liver and gallbladder have to increase the output of their digestive juices that break down fat (known as bile), and that bile acts as a natural laxative. Add a laxative to the aforementioned effect, and it's almost shocking that your colon can ever prevent diarrhea when it encounters a greasy, fatty meal. Eating this kind of processed food even over the span of two weeks can dramatically alter the bacteria in your colon, as we discuss in our article on the gut microbiome.